Microstructural analysis of anhydrite rocks from the Triassic Evaporites, Umbria-Marche Apennines, Central Italy: An insight into deformation mechanisms and possible slip systems

Anhydrite-rich layers within foreland fold and thrust belts are frequently the weakest horizon of the sequence. Characterizing the microstructural textures of anhydrite is therefore important for interpreting the larger scale deformation history of these rocks. The microstructure of naturally deformed anhydrite from the Triassic Evaporites of the Umbria-Marche Apennines, Italy has been analyzed using electron backscatter diffraction (EBSD). This technique has been used to measure the misorientation across distorted grains and the crystallographic preferred orientation (CPO) of the samples. The reference frame of the standard space group Cmcm was used to represent the structure of anhydrite during this analysis which is different to that used in previous studies. Some grains show significant intracrystalline distortions (4 –18° misorientation). Lines of measurements across distortions showed a dispersion of crystallographic data around a clear misorientation axis [010]. These data are combined with the trace orientations of distortions to confirm that the (100)[001] slip systems previously recognized also best explains grain distortions. These interpretations are based on, and consistent with, a tilt boundary model containing edge dislocations. CPO data are also interpreted as a likely (100)[001] slip system, as has previously been recognized. A high degree of twinning within many of the anhydrite grains apparently has little effect on the crystallographic textures observed.